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<h2>Raster data processing in GRASS GIS</h2>
<h3>Raster maps in general</h3>
The geographic boundaries of the raster map are described by the north,
south, east, and west fields. These values describe the lines which bound
the map at its edges. These lines do NOT pass through the center of the
grid cells at the edge of the map, but along the edge of the map itself.
<P>
As a general rule in GRASS:
<ol>
<li> Raster output maps have their bounds and resolution equal to those
of the current region.
<li> Raster input maps are automatically cropped/padded and rescaled
(using nearest-neighbour resampling) to match the current region.
<li> Raster input maps are automatically masked if a raster map named
MASK exists.
</ol>
There are a few exceptions to this:
<tt>r.in.*</tt> programs read the data cell-for-cell, with no resampling. When
reading non-georeferenced data, the imported map will usually have its
lower-left corner at (0,0) in the location's coordinate system; the user
needs to use <a href="r.region.html">r.region</a> to "place" the imported map.
<P>
Some programs which need to perform specific types of resampling (e.g.
<a href="r.resamp.rst.html">r.resamp.rst</a>) read the input maps at
their original resolution then do the resampling themselves.
<P>
<a href="r.proj.html">r.proj</a> has to deal with two regions (source
and destination) simultaneously; both will have an impact upon the
final result.
<h3>Raster import and export</h3>
The module <a href="r.in.gdal.html">r.in.gdal</a> offers a common
interface for many different raster formats. Additionally, it also
offers options such as on-the-fly location creation or extension of
the default region to match the extent of the imported raster map.
For special cases, other import modules are available. Always the full
map is imported.
<P>
For importing scanned maps, the user will need to create a
x,y-location, scan the map in the desired resolution and save it into
an appropriate raster format (e.g. tiff, jpeg, png, pbm) and then use
<a href="r.in.gdal.html">r.in.gdal</a> to import it. Based on
reference points the scanned map can be recified to obtain geocoded
data.
<P>
Raster maps are exported with <a href="r.out.gdal.html">r.out.gdal</a>
into common formats. Also <a href="r.out.bin.html">r.out.bin</a>,
<a href="r.out.vtk.html">r.out.vtk</a> and other export modules are
available.
<h3>Metadata</h3>
The <a href="r.info.html">r.info</a> module displays general information
about a map such as region extent, data range, data type, creation history,
and other metadata.
Metadata such as map title, units, vertical datum etc. can be updated
with <a href="r.support.html">r.support</a>. Timestamps are managed
with <a href="r.timestamp.html">r.timestamp</a>. Region extent and
resolution are mangaged with <a href="r.region.html">r.region</a>.
<h3>Raster map operations</h3>
GRASS raster map processing is always performed in the current region
settings (see <a href="g.region.html">g.region</a>), i.e. the current
region extent and current raster resolution is used. If the resolution
differs from that of the input raster map(s), on-the-fly resampling is
performed (nearest neighbor resampling). If this is not desired, the
input map(s) has/have to be resampled beforehand with one of the dedicated
modules.
<P>
If a raster map named "MASK" exists, most GRASS raster modules will operate
only on data falling inside the masked area, and treat any data falling
outside of the mask as if its value were NULL. The mask is only applied
when <em>reading</em> an existing GRASS raster map, for example when used
in a module as an input map. (see <a href="r.mask.html">r.mask</a>)
<h3>Raster map statistics</h3>
A couple of commands are available to calculate local statistics
(<a href="r.neighbors.html">r.neighbors</a>), and global statistics
(<a href="r.statistics.html">r.statistics</a>, <a href="r.surf.area.html">r.surf.area</a>,
<a href="r.sum.html">r.sum</a>). Profiles and transects can be generated
(<a href="d.profile.html">d.profile</a>, <a href="r.profile.html">r.profile</a>,
<a href="r.transect.html">r.transect</a>) as well as histograms
(<a href="d.histogram.html">d.histogram</a>) and polar diagrams
(<a href="d.polar.html">d.polar</a>).
Univariate statistics (<a href="r.univar.html">r.univar</a>) and
reports are also available (<a href="r.report.html">r.report</a>,<a
href="r.stats.html">r.stats</a>, <a href="r.volume.html">r.volume</a>).
<h3>Raster map algebra and aggregation</h3>
The <a href="r.mapcalc.html">r.mapcalc</a> command provides raster map
algebra methods.
The <a href="r.resamp.stats.html">r.resamp.stats</a> command resamples raster
map layers using various aggregation methods, the <a href="r.average.html">r.average</a>
command aggregates one map based on a second map.
<a href="r.resamp.interp.html">r.resamp.interp</a> resamples raster map layers using interpolation.
<h3>Raster map resampling and interpolation methods</h3>
GRASS offers various raster resampling and interpolation methods:
<ul>
<li> Resampling with nearest neighbor, bilinear, and bicubic method
(<a href="r.resamp.interp.html">r.resamp.interp</a>)</li>
<li> Inverse distance weighted average (IDW) interpolation
(<a href="r.surf.idw.html">r.surf.idw</a> and <a href="r.surf.idw2.html">r.surf.idw2</a>)</li>
<li> Regularized spline with tension (RST) interpolation 2D
(<a href="r.resamp.rst.html">r.resamp.rst</a>)</li>
<li> Bilinear interpolation (<a href="r.bilinear.html">r.bilinear</a>)</li>
<li> Interpolating from contour lines (<a href="r.contour.html">r.contour</a>)</li>
</ul>
<h3>Hydrologic modeling toolbox</h3>
Watershed modeling related modules are
<a href="r.basins.fill.html">r.basins.fill</a>,
<a href="r.water.outlet.html">r.water.outlet</a>,
<a href="r.watershed.html">r.watershed</a>, and
<a href="r.terraflow.html">r.terraflow</a>.
Water flow related modules are
<a href="r.carve.html">r.carve</a>,
<a href="r.drain.html">r.drain</a>,
<a href="r.fill.dir.html">r.fill.dir</a>,
<a href="r.fillnulls.html">r.fillnulls</a>,
<a href="r.flow.html">r.flow</a>, and
<a href="r.topidx.html">r.topidx</a>.
Flooding can be simulated with <a href="r.lake.html">r.lake</a>.
Hydrologic simulation model are available as
<a href="r.sim.sediment.html">r.sim.sediment</a>,
<a href="r.sim.water.html">r.sim.water</a>, and
<a href="r.topmodel.html">r.topmodel</a>.
<h3>Raster format</h3>
Raster data can be stored in GRASS as a 2D integer grid, 2D floating
point grid (single or double precision), or as a 3D floating point
grid (single or double precision). The internal raster format is
architecture independent and portable between 32bit and 64bit
machines. Internally, the integer format is called CELL, single
precision floating point is called FCELL, double precision floating
point is DCELL, and 3D raster is called GRID3D. The choice of the
integer or floating point data depends on the application.
<p>
GRASS distinguishes NULL and zero. When working with NULL data, it is
important to know that operations on NULL cells lead to NULL cells.
<h3>See also</h3>
<ul>
<li><a href=vectorintro.html>Introduction to GRASS vector map processing</a></li>
<li><a href=raster3dintro.html>Introduction to GRASS 3D raster map (voxel) processing</a></li>
</ul>
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